There are a number of ways to achieve better performance from an integrated circuit. One way is to reduce the impedance of wire routes, thus enabling circuit signals to propagate more quickly. The impedance of wire routes may be reduced, for example, by minimizing the impedance of wire routes may be reduced, for example, by minimizing the heights of contacts which couple an integrated circuit (IC) to its environment (e.g., by minimizing the heights of contacts provided on an integrated circuit package). The impedance of wire routes may also be reduced by minimizing the lengths of package signal routes. Package signal routes are the means by which contacts on an IC are coupled to contacts on an integrated circuit package. Yet another way to reduce the impedance of wire routes is to provide sufficient separation and/or insulation between package wire routes and/or package contacts.
Integrated circuit packages which reduce the impedance of wire routes by minimizing the heights of contacts and lengths of package signal routes include the ball grid array (BGA) package, the solder column interposer (SCI) package, and the land grid array (LGA) package. An integrated circuit package which reduces the impedance of wire routes by providing better insulation and/or separation between package wire routes and/or package contacts is the ceramic package. Common ceramic packages include the ceramic BGA (CBGA) and the ceramic LGA (CLGA).
Unlike a BGA or SCI package, which is typically soldered to a printed circuit board, an LGA package is typically mounted to a printed circuit board by means of a socket. Since the contacts of an LGA package are more or less flush with the package's bottom surface, an LGA package may be inserted into or removed from its socket with a relatively low insertion force. Thus, LGA packages are well suited for applications in which an integrated circuit is likely to be removed and replaced for purposes of upgrade, troubleshooting, repair, et cetera.
LGA packages such as the CLGA package are advantageous in that they provide greater second level reliability. Also, their strength and rigidity allow them to be inserted into and removed from sockets many times with only a low probability of wear, stress fractures and the like.
One problem with LGA packages (including the subset of CLGA packages) is that they are difficult to align with respect to the environments in which they operate. Often, an LGA package is aligned by means of its edges. For example, the socket illustrated in FIG. 3 uses a number of spring clips to apply pressure to the edges of an LGA package as it is inserted into the socket, thereby ensuring that the contacts of the LGA package are centered over the socket's own contacts.
While many factors contribute to alignment problems, two are particularly notable. First, the edges of an LGA package are not always true, and do not always bear a precise relationship to the LGA package's contact pads. For example, consider an LGA contact pattern which is applied to a ceramic package base. The ceramic package base may be formed by cutting it from a green sheet and then firing it at a high temperature so that it cures. During the firing process, the base is subject to shrinkage which can result in 1) the base becoming out of square, and 2) the base having skewed, curved or wavy edges. Since no two bases shrink in precisely the same way, it is difficult to design a socket such that each and every LGA package will fit into it the same way.
A second problem which contributes to the misalignment of an LGA package within a socket is that the contact pads which are applied to LGA packages are becoming smaller, and are achieving a finer pitch. As a result, the tolerance for error as an LGA package is aligned in a socket is decreasing.
One way in which the above alignment problem has been addressed is by attaching corner solder balls to the surface of an LGA package on which its contact pads are located. An attempt is made to attach the solder balls at fixed locations with respect to the package's contact pads. Holes for receiving the solder balls are then drilled in a socket at fixed locations with respect to the socket's contact pads (i.e., with the holes having the same relationship to the socket's contacts as the solder balls have to the package's contacts). When an LGA package is then inserted into the socket, the socket's spring clips or other alignment means hopefully align the LGA package so that the solder balls at least approximately fit into their corresponding alignment holes in the socket. The curvatures of the alignment balls bearing on the circumferences of the holes then draw the LGA package into its fully aligned position as the LGA package is pressed into the socket.